The present invention relates to a method and apparatus for characterizing films grown on a silicon substrate using infrared spectroscopy and, more particularly, to characterization of ultra thin silicon oxide films thermally grown on silicon using mirror-enhanced polarized reflectance Fourier Transform Infrared (MEPR-FTIR) Spectroscopy.
The timely achievement of evolving requirements of the National Technology Roadmap for Semiconductors requires a paradigm shift in the role of metrology from off-line sampling to on-line control. Future integrated circuit (IC) technologies will use thinner gate dielectrics. Therefore, non-destructive in-situ probing and characterization of ultrathin ( less than 50 xc3x85) dielectric layers such as SiO2, Si3N4, SiNxOy and even Ta2O5 in real time is highly desirable.
As a non-destructive optical characterization technique, Fourier Transform InfraRed (FTIR) Spectroscopy and Ellipsometry offer sensitive, non-contact techniques that provide surface information as well as bulk material information. Other sensitive and convenient characterization methods include contact angle measurement where the contact angles of water or other solvents reflect the surface criterion directly; that is, surface free energy. FTIR spectroscopy has been used successfully in detecting interstitial oxygen, nitrogen, boron and other impurities in silicon wafers as well as probing adsorbed species on semiconductor surfaces. Since fabrication-compatible FTIR systems have already been used in measuring epitaxial layer thickness, their capabilities in probing and characterizing gate dielectrics are expected to expand in future technologies.
Historically, FTIR spectroscopy had been used to measure film properties via the modes of interferometry, transmission, attenuated total reflectance (ATR) and reflectance. Except for ATR, which can be used for ultrathin film characterization but is not preferred for in-situ applications and requires specially prepared substrates, all others are more effective for relatively thick dielectric layers. A study on silicon oxides down to 6 xc3x85 was recently reported to use dynamically aligned FTIR spectroscopy in single pass external transmission geometry (See K. T. Queeny et al., J. Appl. Phys. 87, 1322 (2000) and B. B. Stefanov et al., Phys. Rev. Lett. 81, 3908 (1998)). For the integration and use of FTIR into the front end characterization metrology of current and future developed ultra-thin gate dielectrics, the development of in-situ FTIR modes for routine probing and characterization of films a few monolayers thick would be desirable.
Ellipsometry has been used for thin film, surface and bulk material characterization of dielectric films typically thicker than about 30 xc3x85. For dielectric films thinner than 30 xc3x85, further work has been conducted by the National Institute of Standards and Technologies and various tool manufacturers in order to enhance the accuracy and repeatability so that widely accepted standards can be set up. Taking advantage of Fourier transform spectroscopy over the infrared range, infrared ellipsometry has recently shown the ability of probing the film morphology from the dielectric function, and the microstructure from the vibrational absorption.
Reflectance IR has been related to thickness characterization because of its in-situ adaptability. However, because of the relatively weak IR intensity it is seldom used in films less than 50 xc3x85 thick.
The sensitivity and selectivity of double modulation FTIR reflection absorption spectroscopy for absorbing species on a reflecting surface has been reported to provide adequate signal-to-noise in a short time; one such application has been the in situ analysis of low-temperature plasma-enhanced chemical vapor deposition of SiO2 films on silicon, and aluminum substrates. (see e.g., Koller et al., J. Appl. Phys. 64, 4704 (1988)). It has also been previously reported that with attachment of a reflective mirror to a single polished or double polished wafer, the Sixe2x80x94O bond at about 1250 cmxe2x88x921 of a thin chemically oxidized layer on Si(111) surfaces could be detected (see e.g., Ohshina et al., Interface control of electrical, chemical, and mechanical properties: Symposium held Nov. 29-Dec. 3, 1993, Boston, Mass., U.S.A. (Materials Research Society, Pittsburgh, 1994, p. 413). Here, however, an air gap between the attached mirror and sample wafer was reported as the reason for the observed IR intensity, the intensity of the IR peaks was weak even with a very sensitive detector (i.e., MCT) and the probing was performed ex-situ.